This invention relates generally to a method and apparatus for electronically assessing the condition of an electrochemical cell or battery while it is in the process of being charged or of supplying power to an active load. More particularly, it relates to special "noise-immune" circuitry for measuring a particular component of a cell/battery's incremental conductance or resistance--a component that has been found by the present inventor to be closely related to the cell/battery's cranking power and energy capacity. Using principles disclosed herein, these measurements can be performed in the presence of very large, time-varying, "noise" signals such as those introduced by alternators, unfiltered chargers, and continuously-switching loads.
Because of its high degree of noise immunity, the disclosed invention can be used to assess the condition of an automotive battery while it is actually being charged by the car's alternator; of an electric vehicle's propulsion battery while the vehicle is being driven; or of the cells of a stationary battery--a battery of the type employed in a telephone central office or an uninterruptible power system (UPS)--while the battery is "on line." The disclosed principles can be advantageously utilized in a variety of applications such as, e.g., a dashboard mounted instrument (e.g., an electric car's "fuel gauge"), a portable hand-held test instrument, or a permanently installed system for remotely monitoring a group of stationary cells.
In order to measure incremental resistance or impedance, one passes a time-varying current through a cell/battery and observes an appropriate component of the resulting time-varying voltage developed across it. Incremental conductance is measured in the opposite manner. One places a time-varying voltage across a cell/battery and observes the appropriate component of time-varying current passing through the cell/battery. In either type of measurement, a problem arises if there are already time-varying currents and voltages (i.e., "noise") present. Such signals, when present during measurement, can degrade accuracy and may introduce serious errors by causing amplifier saturation. Unfortunately, spurious time-varying signals are common occurrences for cells/batteries undergoing either "float" or high-rate charging, or supplying power to an "active" load. Under such circumstances, time-varying battery currents frequently result from imperfect filtering of the battery charger's rectifier or from fluctuations in the current drawn by the load.
One approach to solving the problem of noise introduced by a charger or load is to simply remove the battery from service during measurement of its incremental parameters. This has, in fact, been done innumerable times with satisfactory results. Many cases arise, however, in which it is not desirable or even feasible to take the cell/battery "off line."
A second approach is described in the prior art and is the approach followed by DeBardelaben (S. DeBardelaben, "Determining the End of Battery Life", INTELEC 86, Toronto, Canada, pp. 365-368), as well as by Burkum and Gabriel in U.S. Pat. No. 4,697,134. Their approach is to choose the measurement frequency to be different from any frequencies that are otherwise present in the charger/load circuit and to then use filters to separate the measuring signal from the spurious signals. This solution to the problem is likewise not entirely satisfactory since it assumes prior knowledge of the spurious signal frequencies and requires that the measurement frequency be dictated by the characteristics of the charger/load circuit rather than by requirements of the cell/battery.
A third approach has been described by Robinson in PCT International Publication Number WO 93/22666. Robinson's approach is to use the noise signal itself as a source of broad-band excitation. The noise voltage across a cell and the noise current through the cell are both measured over time, and the frequency-dependent complex impedance of the cell is determined by taking the Fourier transform of their ratio. This solution to the problem is also not entirely satisfactory since it requires that broad-band noise be present to obtain accurate results. Such noise may be characteristic of batteries in service in telephone central offices but is certainly not characteristic of batteries undergoing high-rate charging from poorly-filtered power supplies. Furthermore, such measurements are impossible when the noise is insufficient or non-existent.
A fourth approach is the use of noise cancellation apparatus of the type disclosed by Champlin in U.S. Pat. No. 5,343,380. Such apparatus extends the utility of the dynamic conductance battery testing apparatus disclosed previously by Champlin in U.S. Pat. Nos. 3,873,911, 3,909,708, 4,816,768, 4,825,170, 4,881,038, 4,912,416, and 5,140,269 by permitting the measurements to be made "on line." However, this approach requires that auxiliary apparatus be used in addition to the measuring instrument itself.